The present invention relates to a method of melting a batch of raw materials using a burner placed in the crown of a furnace.
Many industrial processes include a step of melting raw materials, especially for producing materials that do not exist in the natural state, or that exist in the natural state in insufficient quantities or with a low level of purity. Metals and alloys are thus smelted in metallurgy. Likewise, glasses, frits and enamels are produced on an industrial scale by melting vitrifiable batches. In particular, industrial plants for manufacturing flat glass, especially of the soda-lime type, comprise melting furnaces of very large capacity and dimensions.
In the melting field, a mixture of raw materials, the proportions of which are determined according to the material that it is desired to obtain, is called a “batch”. The batch is generally in the form of a substantially homogeneous powder, which is relatively dry and well separated.
To obtain a high output of molten material, a large-capacity furnace running continuously is used. The batch is continuously introduced into the furnace tank on a first side of the furnace and the molten material is discharged, also continuously, on a second side of the furnace, generally opposite the first side. One of the major challenges in installations of this type is to maintain continuity and stability of the melting conditions, from which, particularly, homogeneity of the molten material obtained results.
A charger is used to pour the batch into the tank on top of the fill level of the tank, in the form of a curtain of specified width. The flow of batch thus charged is approximately constant and uniformly distributed over the entire charging width. Given that the charged batch is less dense than the molten material, the batch, which is still pulverulent, stagnates on the surface of the melt. A batch layer that floats on the melt is thus formed. This layer is pushed slowly toward the downstream end of the furnace by the permanent flow of batch that continues to be poured in.
When the batch layer reaches a point in the tank where the temperature is high enough, the batch contained in the layer melts along a well-defined front. This melting front, usually called the “slope”, is the boundary of the batch layer. In general, the shape and orientation of the melting front result from complex mechanisms that are poorly controlled. Convection of the molten liquid within the melt and the gas streams above the melt form part of these mechanisms.
Combustion heating is often used for such furnaces. To do this, one or more burners are placed above the furnace tank. The stability of the melting conditions then depends critically on many parameters, such as the arrangement of the burners, their number, the proportion of fuel to oxidizer used, the distance between the flame(s) of the burner(s), the fill level of the tank, etc.
When a burner of a type described in either of European Patent Applications 1 319 150 and 0 748 981 is used, a narrow flame is obtained. When this is directed toward the batch layer, a melting front is obtained in a limited flame-impact region. When the furnace is operating in a steady state, the melting front of the batch layer then has the shape of a V open toward the downstream end of the furnace. Different batch portions are therefore melted at locations in the furnace that correspond to different levels of advance along the charging direction. This causes deviations in the batch melting conditions. Furthermore, clumps of batch collapse randomly along the batch front, suddenly dropping into the melt. The slope is therefore eroded irregularly, causing uncontrolled movements of the melting front. The operation of the furnace is therefore unstable.
It is possible to place, perpendicular to the charging direction, several burners identical to the previous one in order to push the entire melting front into an upstream part of the furnace. The number of burners used makes such a solution expensive and difficult to implement, especially because of the frequency of the maintenance work on the burners that is therefore necessary.